Photochemistry in the vitamin D field has played a central role in the development of molecular photochemistry. Especially noteworthy have been the early contributions of Havinga1 and of Dauben2 and their coworkers. Havinga's principle of the nonequilibration of excited rotamers (NEER) was introduced to explain the excitation wavelength dependence of the photoproducts of the trienes previtamin D and tachysterol, Scheme 1.3 The NEER principle was based on the idea, inspired by Hückel MO theory, that excitation of conjugated polyenes and related molecules to their lowest singlet or triplet excited states tends to reverse double/single bond character preventing equilibration of ground state conformers. Thus, the ground state conformer equilibrium compositions, the individual spectra and excited state energies of the conformers and the conformer-specific photochemical properties of the conformers control observed product quantum yields and distributions. The dependence of acyclic 1,3-diene photodimer distributions4 and trans/cis photostationary states5,6 on the triplet excitation energy of the sensitizer demonstrated by Hammond and coworkers, soon thereafter, provided dramatic confirmation of this principle, FIG. 1 and Schemes 2 and 3. Ground state conformer controlled photochemical ring opening of a substituted 1,3-cyclohexadiene to isomeric conjugated trienes was first reported in the case of α-phellandrene, Scheme 4.7




Demonstrations of photophysical manifestations of NEER include the resolution of the fluorescence of all-trans-1,6-diphenyl-1,3,5-hexatriene into s-trans,s-trans and s-cis,s-trans contributions8 and analogous resolutions of fluorescence and absorption spectra of styrylarene conformers.9,10 The conformer-specific adiabatic cis trans photoisomerizations of 2-styrylnaphthalene11 (cis- and trans-NPE) and 2-styrylanthracene12 are especially striking examples. In each case, the more extended s-trans-like conformer undergoes adiabatic cis  trans photoisomerization whereas the less extended s-cis-like conformer undergoes selective photocyclization, Scheme 5.13,14 In this sense, these cis-diarylethenes can be regarded as true previtamin D mimics.

Adherence to the NEER principle is expected in molecules with relatively short-lived excited states in which the electronic excitation is delocalized extensively over the conjugated Π-system. In such molecules torsional barriers about essential ground state single bonds are enhanced in the lowest excited states. Conversely, molecules that have long excited state lifetimes and in which there is pronounced localization of electronic excitation in parts of the Π-system are likely to violate the NEER principle. Alkenyl-substituted anthracenes provide well-documented examples.9,15-17 
In recent years, it has been suggested that not all the excitation wavelength dependence of quantum yields in the vitamin D field can be accounted for by the NEER principle. Below are considered the competing mechanisms that have been proposed to account for photochemical observations and describe some of the strategies that have been employed to improve the photochemical production of the previtamins from the provitamins. Optimization of the previtamin yields improves vitamin yields as the latter are formed thermally from the previtamins via 1,7-suprafacial hydrogen shifts.18 Readers interested in the rich photochemistry leading to over-irradiation products should consult previous reviews.1a,2b 